How long does an AC unit last?

The 12–15 year range is an average, not a guarantee in either direction. An AC that gets annual maintenance, has clean coils, and was installed with correct refrigerant charge and airflow tends to sit at the long end of that range or beyond. One that's never been serviced, has run dirty for years, or was undersized or oversized at installation (both cause the compressor to work harder than it should) often fails well before 12.

The practical question isn't really "how long should mine last" — it's "is mine past the point where repair still makes sense." Past year 10, a major repair (compressor, coil) usually gets compared honestly against replacement, since you'd be investing significant money into a unit that's already most of the way through its expected life.